A satellite positioning system receives information sent from a plurality of satellites going around the earth's orbit, measures the distance between the satellite positioning system and each satellite and calculates the current location of a receiving side apparatus. GPS, established by the United States Department of Defense, is a typical satellite positioning system, and provides a plurality of satellites referred to as “GPS satellites.”
A GPS satellite performs spectrum spreading processing using predetermined PRN (Pseudo Random Noise) codes with respect to signals to be sent out. That is, a mobile communication terminal can acquire original signals by performing despreading processing of the signals sent out from these GPS satellites (hereinafter referred to “GPS signals”) using the matching PRN codes. Then, information about the current location of this mobile communication terminal and the current time can be acquired by carrying out processing such as message synchronization, ephemeris collection and PVT (Position, Velocity, Time) calculation.
Generally, a positioning apparatus that utilizes satellites receives four or more satellite signals at the same time, tracks the spreading codes by capturing the carrier waves and performs spectrum despreading processing to demodulate navigation data from the satellite signals. Further, by calculating the time a satellite transmitted a signal based on navigation data and finding a pseudo distance on a per satellite basis (i.e. the time a satellite signal took to reach the positioning apparatus), the positioning apparatus's location is determined based on the pseudo distance.
Currently, GPS receivers are widely utilized in various fields, such as car navigation systems, mobile telephones, airplane control and measurement of diastrophism. Although a GPS receiver receives the 1575.42 MHz signal referred to as “L1,” the 1227.6 MHz signal and 1176.45 MHz signal will additionally become available as the second civilian signal (L2C) and third civilian signal (L5C), respectively.
Further, the Galileo system is planed in Europe to construct a satellite positioning system using the same number of satellites as in GPS. A plurality of frequency signals are also prepared for this Galileo system, and, consequently, by utilizing a plurality of these signals, it is possible to realize better performance than conventional performance (making a range of a receiving area wider and improving the accuracy of a positioning system).
Patent Document 1 discloses a positioning satellite receiving apparatus that has a plurality of receiving systems, and that switches receiving characteristics to set in a positioning receiving configuration with a plurality of receiving bands.
FIG. 1 is a functional block diagram showing a conventional positioning receiving apparatus having a plurality of receiving bands.
In FIG. 1, a high frequency signal received at receiving antenna 11 is amplified in received signal amplifier 12 formed with an LNA (Low Noise Amplifier), and then is inputted to I-ch (I-channel) mixer 13 and Q-ch (Q-channel) mixer 14 constituting a quadrature mixer.
90 degree phase shifter 15 supplies a local signal as is to mixer 13, from local oscillator 16 that performs oscillation at the same frequency as the carrier frequency, and shifts the local signal from local oscillator 16 90 degrees and supplies the local signal to mixer 14. As a result, mixer 13 converts the output signal of received signal amplifier 12 into a frequency-domain I signal, which is the baseband signal of the in-phase component, and outputs the frequency-domain I signal, and converts the output signal of received signal amplifier 12 into a frequency-domain Q signal, which is the baseband signal of the quadrature component, and outputs the frequency-domain Q signal.
These I signal and Q signal are subjected to processing for removing low frequency components through first I low pass filter 17 and first Q low pass filter 19, which are the first I and Q baseband filters, and received as input in digital processing section 21 through A/D converters (not shown), and are subjected to processing for removing low frequency components through second I low pass filter 18 and second Q low pass filter 20, which are the second I and Q baseband filters, and received as input in digital processing section 21 through A/D converts (not shown).
Digital processing section 21 performs digital signal processing of the first I and Q signals and second I and Q signals formed with digital signals, to extract data. The extracted data is outputted to CPU 22. CPU 22 is a controlling section that selects satellites to scan, performs tracking control of received signals and acquires navigation messages transmitted from satellites.
Patent Document 1 discloses a positioning satellite receiving apparatus that has a plurality of receiving systems, and switches receiving characteristics to set in a positioning receiving configuration having a plurality of receiving bands.
FIG. 2 is a block diagram showing a schematic configuration of the satellite positioning signal receiving apparatus having two receiving processing systems disclosed in Patent Document 1.
In FIG. 2, satellite positioning signal receiving apparatus 30 is constituted by: antenna 31 that receives signals from satellites; RF amplifying section 32 that amplifies the signals received at antenna 31; reference crystal oscillator 33; digital processing section 34 that performs signal processing; operation receiving section 35 that receives operations from the user; and two receiving processing systems, that is, the first receiving processing system formed with mixer section 41, local oscillator 42, IF amplifying section 43 and IF filter section 44, and the second receiving processing system formed with mixer section 51, local oscillator 52, IF amplifying section 53 and IF filter section 54.
Digital processing section 34 is constituted by microcomputers and the like, and executes various control processings for data demodulation. This digital processing section 34 is connected to local oscillators 42 and 52 and IF filter sections 44 and 54, and switches characteristics of these components to set separately. Local oscillators 42 and 52 are connected to one reference crystal oscillator 33, and generate frequencies based on commands from digital processing section 34 using the reference frequency acquired from reference crystal oscillator 33.
According to the above configuration, for example, a satellite positioning signal, which is the target to receive, is selected based on an operation received in operation receiving section 35 from the user or based on the received states of satellite positioning signals from digital processing section 34, and the characteristics of local oscillators 42 and 52 and IF filter sections 44 and 54 are switched to set.    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-207888